System Comprising an Integrated Waveguide-Coupled Optically Active Device and Method of Formation

ABSTRACT

Integrated-optics systems are presented in which an optically active device is optically coupled with a silicon waveguide via a passive compound-semiconductor waveguide. In a first region, the passive waveguide and the optically active device collectively define a composite waveguide structure, where the optically active device functions as the central ridge portion of a rib-waveguide structure. The optically active device is configured to control the vertical position of an optical mode in the composite waveguide along its length such that the optical mode is optically coupled into the passive waveguide with low loss. The passive waveguide and the silicon waveguide collectively define a vertical coupler in a second region, where the passive and silicon waveguides are configured to control the distribution of the optical mode along the length of the coupler, thereby enabling the entire mode to transition between the passive and silicon waveguides with low loss.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is claims priority to U.S. Provisional PatentApplication Ser. No. 62/961,348, filed Jan. 15, 2020, entitled “SystemComprising an Integrated Waveguide-Coupled Optically Active Device andMethod of Formation,” (Attorney Docket 3218-001PR1), which isincorporated by reference. If there are any contradictions orinconsistencies in language between this application and one or more ofthe cases that have been incorporated by reference that might affect theinterpretation of the claims in this case, the claims in this caseshould be interpreted to be consistent with the language in this case.

TECHNICAL FIELD

The present disclosure relates to heterogeneous integration ofcompound-semiconductor structures on silicon substrates in general, and,more particularly, to the integration of compound-semiconductor activeand passive photonic elements with silicon for forming semiconductorwaveguides, active electronic devices, and/or active and passivephotonic devices.

BACKGROUND

Silicon photonics promises relatively low-cost solutions for manyphotonic applications. However, historically, most silicon-photonicssystems cannot generate light on the chip. As a result, there areongoing efforts toward enabling on-chip light generation, amplification,modulation, etc. via heterogeneous silicon-photonic integration.

Some success in providing on-chip light generation, amplification,modulation, etc., in silicon-photonics systems has been achieved byintegrating compound-semiconductor material onto a substrate containingsilicon waveguides via direct bonding and subsequent formation of activecompound-semiconductor waveguides above silicon waveguides.

In some alternative prior-art approaches, discrete optically activedevices (e.g., lasers, amplifiers, modulators, etc.) are completelyformed separately and optically coupled with a photonic integratedcircuit (PIC) through, for example fiber coupling the device and thePIC, flip chip bonding the device onto the PIC substrate and opticallycoupling it with a silicon waveguide on the PIC via a silicon gratingcoupler, or employing conventional packaging methods wherein light iscoupled between the discrete device and a silicon waveguide on the PIC.

Unfortunately, the cost and complexity of such approaches have, thusfar, limited their use.

A platform that enables cost-effective formation of a heterogeneoussilicon photonic system in a cost-effective manner remains, as yet,unmet in the prior art.

SUMMARY

The present disclosure is directed toward integrated-optics systems thatinclude an optically active device optically coupled with a siliconwaveguide via a passive compound-semiconductor waveguide, where theoptically active device, the passive waveguide, and the siliconwaveguide reside on a common substrate. Embodiments in accordance withthe present disclosure are particularly well suited for use indistributed-feedback lasers, mode locked lasers, photonic integratedcircuits, external-cavity mode locked lasers, loop-mediated isothermalamplification devices, and the like.

An advance is made over the prior art by employing a passivecompound-semiconductor waveguide as a transition element between anoptically active device and a silicon waveguide. The optically activedevice includes active material that is disposed on and opticallycoupled with a compound semiconductor coupling waveguide. The activematerial resides completely in a first region of a substrate. Thesilicon waveguide is wholly contained in a second region of thesubstrate. The passive waveguide resides in a transition region betweenthe first and second regions and extends into these regions such thatthe passive waveguide is optically coupled with each of the opticallyactive device and silicon waveguide. The active material, the couplingwaveguide, the passive waveguide, and the silicon waveguide configuredto dictate the vertical location and lateral confinement of opticalenergy at each point along the length of the system.

The use of a compound-semiconductor passive waveguide as a transitionelement to optically couple an optically active device and a siliconwaveguide affords significant advantages over the prior art, such as:

-   -   i. independent control over the performance of the optically        active device and the coupling efficiency of a light signal into        the silicon waveguide, thereby enabling each to be substantially        optimized without degrading the other; or    -   ii. transition of a light signal between the optically active        device and the silicon waveguide with less optical loss than can        be achieved in the prior art; or    -   iii. in monolithically integrated systems, facilitated epitaxial        growth of the optically active device by virtue of the presence        of a compound semiconductor layer from which the passive        waveguide is formed in the first region;    -   iv. in heterogeneously integrated systems, facilitated bonding        of nascent optical-device material on the common substrate by        virtue of the presence in the active region of a compound        semiconductor layer, from which the passive waveguide is formed;        or    -   v. any combination of i, ii, iii, and iv.

Further advance over the prior art is made by defining a couplingwaveguide and a passive waveguide from contiguous portions of acompound-semiconductor layer and forming a composite waveguide having arib portion that is an optically active device and a planar portion thatis the coupling waveguide. By tailoring the lateral dimensions of theoptically active device, the size and/or vertical position of an opticalmode of a light signal in the composite waveguide can be controlled. Asa result, the optical mode can be located at least partially in theoptically active device at one location and forced substantiallycompletely into the passive waveguide as the light signal propagates toa second location.

An illustrative embodiment is a waveguide-coupled optical system havingan active region, an output-coupling region, and a transition region,each of which is disposed on a silicon-on-insulator substrate. Thesystem includes: a quantum-dot laser whose active material is locatedcompletely within an active region, where it is disposed on andoptically coupled with a gallium arsenide coupling waveguide; a siliconwaveguide that resides completely in the output-coupling region; and apassive gallium-arsenide waveguide that resides in the transition regionand extends into each of the active and output-coupling regions where itis optically coupled with the laser and the silicon waveguide,respectively. As a result, the passive waveguide couples optical energygenerated by the laser into the silicon waveguide.

In the active region, the active material is patterned to define atapered region that defines a first coupler that facilitates transfer ofoptical energy generated in the laser structure into a light signalpropagating in the coupling waveguide.

In the output-coupling region, the silicon device layer of thesilicon-on-insulator substrate is patterned to define a siliconwaveguide that functions as a single-mode waveguide for the lightsignal.

In the transition region, the coupling waveguide is patterned to definea single-mode passive waveguide for the light signal. No opticallyactive material or electrical contacts are present in the transitionregion. The passive waveguide extends slightly into the output-couplingregion, where it is tapered to define a second coupler that facilitatestransfer of the light signal into the silicon waveguide.

In some embodiments, the coupling layer includes a lower sub-layer andan upper sub-layer, where the lower sub-layer is doped to facilitateforming electrical contacts. The upper sub-layer is characterized by ahigher refractive index than the lower sub-layer, thereby enabling thelower sub-layer to also function as a lower cladding layer.

In some embodiments, the optically active device is a device other thana laser, such as an optical modulator (e.g., an electroabsorptionmodulator, a phase modulator, etc.), an optical amplifier, a variableoptical attenuator, a photodetector, and the like. In some embodiments,the optically active device includes a quantum element other than aquantum dot, such as a quantum dash, quantum well, a quantum wire, andthe like. In some embodiments, optically active device does not includea quantum element.

In some embodiments, the active region includes an optically activedevice and coupling layer of a different compound semiconductor, such asindium phosphide, indium gallium arsenide phosphide, and the like. As aresult, the passive waveguide also comprises this different compoundsemiconductor.

In some embodiments, a reflector is defined in at least one of thepassive and silicon waveguides to redirect a light signal. In someembodiments, the reflector is defined in the passive waveguide and isconfigured to optically couple with a vertical grating coupler definedin the silicon waveguide.

In some embodiments, an alignment feature is included for passivelyaligning a bulk optical element (e.g., an optical fiber, aphotodetector, a light source, etc.), to one of the passive and siliconwaveguides. In some such embodiments, this alignment feature is asilicon-optical-bench feature.

In some embodiments, the silicon waveguide is not included and thepassive waveguide functions as an optical interface to another opticalelement. In some embodiments, the other optical element and theoptically active device are disposed on the same substrate. In someembodiments, the other optical element is external to the substratecomprising the optically active device. In some such embodiments, thepassive waveguide is configured such that its optical mode issubstantially matched with the optical mode of an external element tomitigate optical coupling loss.

In some embodiments, a spot-size converter is included in the passivewaveguide and/or the silicon waveguide to facilitate optical couplingwith an external element.

An embodiment in accordance with the present disclosure is anintegrated-optics system disposed on a substrate, the system comprisingan optically active device that is optically coupled with a siliconwaveguide, the system including: a first region that selectivelyincludes the optically active device, the optically active device beingdisposed on a coupling waveguide that includes a first portion of afirst layer that comprises a compound semiconductor, wherein theoptically active device and the coupling waveguide collectively define acomposite waveguide that at least partially supports an optical mode ofa light signal; a second region that includes a first waveguide that isa passive waveguide configured to at least partially support the opticalmode, the first waveguide having a first core that comprises a secondportion of the first layer, the first waveguide being optically coupledwith the composite waveguide; and a third region that selectivelyincludes a second waveguide having a second core comprisingsingle-crystal silicon, wherein the second waveguide is configured to atleast partially support the optical mode and is optically coupled withthe first waveguide; wherein the first region, second region, and thirdregion are contiguous.

Another embodiment in accordance with the present disclosure is a methodfor forming an integrated-optics system disposed on a first substrate,the system having a first region, a second region, and a third regionand comprising an optically active device that is optically coupled witha silicon waveguide, the method including: forming a coupling waveguidein the first region, the coupling waveguide including a first portion ofa first layer comprising a compound semiconductor; forming an opticallyactive device on the coupling waveguide, the optically active devicecomprising at least one quantum element, wherein the optically activedevice and the coupling waveguide collectively define a first compositewaveguide that at least partially supports an optical mode of a lightsignal, and wherein the optically active device is located only in thefirst region; forming a first waveguide in a second region, the firstwaveguide being a passive waveguide configured to at least partiallysupport the optical mode, wherein the first waveguide has a first corethat comprises a second portion of the first layer, and wherein thefirst waveguide is optically coupled with the first composite waveguide;and forming a second waveguide in a third region, the second waveguidehaving a second core comprising single-crystal silicon and beingconfigured to at least partially support the optical mode, wherein thesecond waveguide is optically coupled with the first waveguide.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 depicts a schematic drawing of a top view of an illustrativeintegrated-optics system in accordance with the present disclosure.

FIGS. 2A-E depict sectional views of system 100 through lines a-athrough e-e, respectively.

FIG. 3 depicts operations of a method suitable for forming system 100 inaccordance with the illustrative embodiment.

FIG. 4A depicts a cross-sectional drawing of a portion of a sacrificialsubstrate comprising coupling layer 208 and active-material stack 210.

FIG. 4B depicts a cross-sectional view of nascent system 100 aftercoupling layer 208 and silicon device layer 206 have been bonded.

FIG. 5A depicts a schematic drawing of a sectional view of analternative composite waveguide in accordance with the presentdisclosure.

FIG. 5B depicts a schematic drawing of a sectional view of anotheralternative composite waveguide in accordance with the presentdisclosure.

FIG. 6 depicts a schematic drawing of a sectional view of an alternativetransition region in accordance with the present disclosure.

FIG. 7 depicts a schematic drawing of a sectional view of an alternativeembodiment of a coupler for optically coupling a passive waveguide and asilicon waveguide in accordance with the present disclosure.

FIG. 8 depicts a schematic drawing of a cross-sectional view of yetanother alternative coupler in accordance with the present disclosure.

FIGS. 9A and 9B depict schematic drawings of cross-sectional views ofalternative output ports in accordance with the present disclosure.

FIG. 10 depicts a schematic drawing of an alternative system inaccordance with the present disclosure.

FIG. 11 depicts a schematic drawing of another alternative system inaccordance with the present disclosure.

DETAILED DESCRIPTION

For the purposes of the present disclosure, including the appendedclaims, the following terms are defined:

-   -   “disposed on” or “formed on” is defined as is defined as “exists        on” an underlying material or layer with or without intermediate        layers. For example, if a material is described to be “disposed        (or grown) on a substrate,” this can mean that either (1) the        material is in intimate contact with the substrate; or (2) the        material is in contact with one or more layers that reside on        the substrate.    -   optically active device is defined as an electrically coupled        device (i.e., a device comprising electrical contacts for        connecting to external circuitry) in which, in response to an        electrical signal applied to the electrical contacts, either (a)        photons are generated due to the recombination of free carriers        or (b) free-carrier pairs are generated due to the absorption of        photons. Examples of optically active devices include, without        limitation, lasers, optical amplifiers, optical modulators        (e.g., electroabsorption modulators, phase modulators, etc.),        variable optical attenuators, photodetectors, and the like. It        should be noted that an optically active device does not require        inclusion of a quantum-element-containing layer.    -   quantum element is defined as a semiconductor structure that        exhibits a quantum effect. Examples of quantum elements include,        without limitation, quantum dots, quantum wells, quantum-well        layers, quantum dashes, quantum wires, and the like.    -   passive waveguide is defined as a surface waveguide in which        light passes through virtually unperturbed. Passive waveguides        are not operatively coupled with electrical contacts and are not        stimulated to exhibit optoelectronic effects, such conversion of        free carriers into photons or vice versa, optical modulation,        optical amplification, and the like.

For the purposes of this Specification, including the appended claims,the terms “lateral” and “vertical” as used are meant to be relative tothe major surfaces of a substrate on which an integrated-optics systemresides, where the term lateral refers to directions that are parallelto the major surfaces and the term vertical refers to directions thatare normal to the major surfaces. In similar fashion, the term “lower”means more proximate to the substrate and the term “higher” means moredistal from the substrate.

FIG. 1 depicts a schematic drawing of a top view of an illustrativeintegrated-optics system in accordance with the present disclosure.System 100 includes active region 102, transition region 104, andoutput-coupling region 106, which are disposed on substrate 108. Activeregion 102, transition region 104, and output-coupling region 106 arecontiguous regions that collectively define an integrated-optics-based,silicon-waveguide-coupled laser. System 100 comprises optically active(OA) device 110, coupling waveguide 112, passive waveguide 114, andsilicon waveguide 116, which are arranged such that optical energygenerated by OA device 110 propagates as light signal 118 from the laserto port 120.

It should be noted that, although the illustrative embodiment is anintegrated-optics-based, silicon-waveguide-coupled laser wherein lightis generated in an optically active device and propagates to a waveguideport, embodiments in accordance with the present disclosure includesystems wherein a light signal is coupled into a waveguide port andconveyed to an OA device. Furthermore, in some embodiments, no siliconwaveguide is included and port 120 is located in passive waveguide 114.Furthermore, in some embodiments, more than one passive waveguide and/orsilicon waveguide is included.

FIGS. 2A-E depict schematic drawings of sectional views of system 100through lines a-a through e-e, respectively.

FIG. 3 depicts operations of a method suitable for forming system 100 inaccordance with the illustrative embodiment. Method 300 is describedwith continuing reference to FIGS. 1 and 2A-E. Method 300 begins withoperation 301, wherein silicon waveguide 116 is defined on substrate 108in output-coupling region 106.

Substrate 108 is a conventional silicon-on-insulator (SOI) substratecomprising handle substrate 202, buried oxide layer (BOX) 204, andsilicon device layer 206. In the depicted example, handle substrate 202is a conventional silicon wafer, BOX 204 is a layer of thermally grownsilicon dioxide having a thickness that is typically within the range ofapproximately 1-2 microns and, preferably, 2 microns, and silicon devicelayer 206 is a layer of single-crystal silicon having a thickness equalto approximately 220 nm. As will be apparent to one skilled in the artafter reading this Specification, however, substrate 108 can be anysubstrate suitable for use in system 100. Examples of substratessuitable for use in accordance with the present disclosure include,without limitation, glass substrates, compound-semiconductor substrates,bulk silicon substrates, and the like.

Silicon waveguide 116 is a rib waveguide having a rib portion of widthw6. In output-coupling region 106, silicon waveguide is configuredenable the waveguide to support single-mode propagation of light signal118. Silicon waveguide 116 is formed by patterning silicon device layer206 via conventional lithography and etching to define its structure. Insome embodiments, silicon waveguide 116 has a waveguide structure otherthan that of a rib waveguide, such as a channel waveguide, stripwaveguide, ridge waveguide, etc.

At operation 302, coupling layer 208 and active-material stack 210 areadded to substrate 108.

In the depicted example, heterogeneous integration techniques are usedto add coupling layer 208 and active-material stack 210 to substrate108. Heterogeneous integration techniques suitable for use in accordancewith the present disclosure are described in, for example, U.S. Pat.Nos. 9,097,848, 9,910,220, 8,380,033, 8,620,164, each of which isincorporated herein by reference in their entirety.

In accordance with conventional heterogeneous integration, a separatephotonic substrate is formed by epitaxially growing active-materialstack 210 and coupling layer 208 on a sacrificial substrate.

FIG. 4A depicts a schematic drawing of a cross-sectional view of aportion of a photonic substrate in accordance with the illustrativeembodiment. Photonic substrate 400 includes coupling layer 208 disposedon active-material stack 210, which is disposed on sacrificial substrate402.

In the depicted example, sacrificial substrate 402 is a conventionalgallium arsenide wafer and coupling layer 208 is a layer of galliumarsenide having a thickness suitable for supporting single-modepropagation of light signal 118. In some embodiments, at least one ofsacrificial substrate 402 and coupling layer 208 comprises a compoundsemiconductor other than gallium arsenide, such as indium phosphide,indium gallium arsenide, indium gallium arsenide phosphide, and thelike.

Preferably, coupling layer 208 comprises sub-layers 208A and 208B, wheresub-layer 208A has a lower refractive index than that of sub-layer 208B.As a result, sub-layer 208A can function as a cladding layer that servesto substantially confine at least a portion of the optical mode of lightsignal 118 to sub-layer 208B as the light signal propagates throughpassive waveguide 114. In some embodiments, coupling layer 208 includesat least one additional sub-layer distal to sub-layer 208A, where thisadditional sub-layer or sub-layers are configured to function as anupper cladding for sub-layer 208B. In some embodiments, coupling layer208 does not include sub-layers (i.e., it is a homogeneous layer).

Furthermore, in some embodiments, sub-layer 208A is doped to reducecontact resistance for contacts 122 n.

Active-material stack 210 includes the constituent layers of OA device110, including cladding layers, carrier confinement layers, and gainlayer 212. In the depicted example, OA device 110 is a quantum-dot lasercomprising active-material stack 210. In some embodiments, OA device 110is a different optically active device, such as an optical modulator(e.g., an electroabsorption modulator, a phase modulator, etc.), anoptical amplifier, a variable optical attenuator, a photodetector, andthe like.

It should be noted that, although the illustrative embodiment includes again layer comprising a plurality of quantum dots, gain layer 212 caninclude any one or more of a wide variety of quantum elements withoutdeparting from the scope of the present disclosure. Quantum elementssuitable for inclusion in gain layer 212 include, without limitation,quantum wells, quantum-well layers, quantum wires, quantum dashes, andthe like.

In addition, as discussed below, preferably, active-material stack 210also includes sub-layers having different refractive indices to controlthe vertical position of optical mode 216. For example, in someembodiments, active-material stack 210 contains higher aluminum contentwith gallium arsenide in the layers between active layer 212 and its topcontact layer. This provides a lower index of refraction that can forceoptical energy in an optical mode within the material stack downwardtoward coupling waveguide 112. Furthermore, defining the active-materialstack as a ridge or rib also helps force the optical mode toward thecoupling waveguide while serving to laterally contain the optical modeas well.

In some embodiments, a dielectric layer is included between couplinglayer 208 and silicon device layer 206 to act as a lower cladding thatconfines the optical mode toward at least one of the middle portion ofpassive waveguide 114 and the middle portion of active-material stack210.

In some embodiments, active-material stack 210 also includes a regionbetween active layer 212 and coupling waveguide 112 that has higheraluminum content to create a lower index of refraction, thereby forcingthe optical mode upward away from the coupling waveguide. It should benoted that this same layer can also function as an upper cladding forcoupling waveguide 112.

Once active-material stack 210 and coupling layer 208 are complete,photonic substrate 400 is then flipped over and coupling layer 208 isbonded to silicon device layer 206 via direct bonding. In someembodiments, a different bonding technology is used to join couplinglayer 208 and device layer 206, such as plasma bonding, fusion bonding,thermo-anodic bonding, and the like. In some embodiments, an interfacelayer is included between silicon device layer 206 and coupling layer208 to facilitate their bonding.

Once photonic substrate 400 and substrate 108 are bonded, sacrificialsubstrate 402 is removed in conventional fashion.

FIG. 4B depicts a schematic drawing of a cross-sectional view of nascentsystem 100′ after the removal of sacrificial substrate 402.

Although in the depicted example, coupling layer 208 and active-materialstack 210 are added to substrate 108 via heterogeneous bondingtechniques, in some embodiments, they are epitaxially grown on thesubstrate. In such embodiments, coupling layer 208 is grown directly onsilicon device layer 206 via hetero-epitaxial growth, which is followedby epitaxial growth of active-material stack 210 on the coupling layer.

Returning now to method 300, at operation 303, active-material stack 210is patterned to define the lateral dimensions of OA device 110. Itshould be noted that this removes active material completely from eachof transition region 104 and output-coupling region 106. As a result,optically active material is selectively included in active region 102.However, as will be apparent to one skilled in the art, after readingthis Specification, in some embodiments, a substrate has multiple activeregions, each containing corresponding patterns of active-material stack210.

OA device 110 is patterned such that it has nominal width w1 outside ofthe area of coupler 128-1, where the value of w1 is selected tofacilitate lateral confinement of light signal 118 in the active region.In the depicted example, w1 is equal to 2 microns; however, w1 can haveany suitable value. Typically, w1 is within the range of approximately0.5 micron to approximately 4 microns.

At operation 304, coupling layer 208 is patterned in conventionalfashion to define coupling waveguide 112 in active region 102 andpassive waveguide 114 in transition region 104, where the passivewaveguide supports single-mode propagation of light signal 118. Itshould be noted, however, that in active region 102, coupling waveguide112 is not typically a single-mode waveguide for light signal 118.

Coupling waveguide 112 is formed as a rib waveguide. Coupling waveguide112 includes a central ridge portion having width w2 and a planarportion having width w3. In the depicted example, w2 and w3 are 6microns and 100 microns, respectively; however, each of w2 and w3 canhave any suitable value. Typically, the value of w2 is within the rangeof approximately 0.5 micron to 20 microns. As will be apparent to oneskilled in the art, the value of w3 is not typically critical, but isnormally within the range of 25 to 500 microns.

It should be noted that, after operation 304, active-material stack 210and coupling waveguide 112 collectively define composite waveguide 214as a rib waveguide (also sometimes referred to as a strip waveguide),where the active-material stack functions as the projecting ridgeportion of the composite waveguide and the central ridge portion ofcoupling waveguide 112 functions as the planar portion of the ribwaveguide.

In the depicted example, the width, w2, of coupling waveguide 112 isselected such that the coupling waveguide, itself, contributes little orno lateral confinement of optical mode 216 within active region 102. Asa result, the shape and size of optical mode 216 in active region 102 isdetermined primarily by the lateral dimensions of active material stack210. In some embodiments, however, coupling waveguide 112 has a widththat enables the coupling waveguide to provide lateral confinement ofthe optical mode. In some embodiments, the width of coupling waveguide112 is substantially equal to the width of active-material stack 210(i.e., w2=w1 and the coupling waveguide is substantially a channelwaveguide).

Passive waveguide 114 is also formed as a rib waveguide comprising acentral ridge portion having width w4 and a planar region having widthw5. In the depicted example, w4 and w5 are 2 microns and 10 microns,respectively; however, each of w4 and w5 can have any suitable value.Typically, the value of w4 is within the range of approximately 0.5micron to 4 microns. As will be apparent to one skilled in the art, thevalue of w5 is not typically critical, but is normally within the rangeof 0.5 to 100 microns.

Since coupling waveguide 112 and passive waveguide 114 are continuoussegments of coupling layer 208, they are inherently optically coupled.

At operation 304, contacts 122 n are formed outside the central ridgeportion of coupling waveguide 112.

At operation 305, contact 122 p is formed on the top surface of OAdevice 110 to complete the formation of system 100.

It should be noted that, in some embodiments, coupling waveguide is aslab waveguide (i.e., no ridge and planar portions are defined incoupling layer 208 within active region 102). In such embodiments,contacts 122 n can be formed on the top surface of the coupling layer,in vias partially etched down to sub-layer 208A, or in any mannersuitable for making them operatively coupled with OA device 110. In someembodiments, coupling waveguide 112 is formed as a channel waveguide(i.e., no planar portion remains after coupling layer 208 has beenetched to define the waveguide). In such embodiments, device layer is inelectrical contact with sub-layer 208A and contacts 122 n are formed insilicon device layer 206.

Upon completion of system 100, active region 102 selectively includes OAdevice 110 and coupling waveguide 112, output-coupling region 106selectively includes silicon waveguide 116, and transition region 104includes passive waveguide 114. Passive waveguide 114 also extends intoactive region 102 to form coupler 128-1 with coupling waveguide 112 andextends into output-coupling region 106 to form coupler 128-2 withsilicon waveguide 116.

It is an aspect of the present invention that the lateral dimensions ofactive-material stack 210 substantially determine the vertical positionat which optical energy in OA device 110 forms an optical mode, as wellas the shape of that optical mode. As a result, active material stack210 is defined such that it includes a first segment (i.e., gain section124) that is configured to favor optical gain that gives rise to anoptical mode and a second segment (i.e., taper 126-1) that is configuredto force that optical mode into coupling waveguide 112.

FIG. 2A depicts a schematic drawing of a sectional view of OA device 110taken through gain section 124 (i.e., through line a-a shown in FIG. 1).

As seen in FIG. 2A, in gain section 124, the relative values of w1 andw2 give rise to optical mode 216 such that its optical energy iscontained within one continuous region that is substantially centrallylocated in composite waveguide 214. In other words, each of OA device110 and coupling waveguide 112 partially supports optical mode 216.

In some embodiments, coupling waveguide 112 and active-material stack210 are configured such that optical mode 216 extends over a continuousregion that includes at least a portion of each of the active-materialstack, the coupling waveguide, and silicon device layer 206 (i.e.,optical mode 216 is partially supported by each of the active-materialstack, the coupling waveguide, and the silicon device layer).

Coupler 128-1 is a section of active region 102 configured for forcingsubstantially all of the optical energy of light signal 118 located inactive-material stack 210 into coupling waveguide 112 so that it canefficiently couple into passive waveguide 114. Coupler 128-1 includestapers 126-1 and 126-2.

Taper 126-1 is a segment of active-material stack 210 that is configuredto force the optical energy of light signal 118 into coupling waveguide112. Taper 126-1 has length L1 and a width that reduces from w1 to zero(i.e., extinction) along length L1. In the depicted example, L1 is equalto 100 microns; however, it is typically within the range ofapproximately 50 microns to approximately 500 microns. It should benoted that the value of L1 is a matter of design choice and, therefore,it can have any suitable value. Furthermore, in some embodiments, taper126-1 does not taper to extinction but, rather, to a non-zero width(e.g., one micron or less) that is sufficiently narrow to force theoptical energy of light signal 118 from OA device 110 into passivewaveguide 114.

In similar fashion, taper 126-2 is a segment of coupling waveguide 112that is configured to facilitate the transfer of light signal 118 intopassive waveguide 114 as a single-mode signal. Taper 126-2 has lengthL2, over which the width of coupling waveguide 112 changes from w2 to w4(i.e., the width of passive waveguide 114). In the depicted example, L2is equal to 50 microns; however, it is typically within the range ofapproximately 10 microns to approximately 500 microns. It should benoted that the value of L2 is not critical and, therefore, it can haveany value within a wide range.

FIG. 2B depicts a schematic drawing of a sectional view of coupler 128-1as taken through line b-b shown in FIG. 1.

As noted above, the distribution of the optical energy of optical mode216 between active-material stack 210 and coupling layer 208 (i.e., theshape and vertical position of the optical mode) is based on therelationship of w1 and w2, which change along length L1 of taper 126-1.Tapers 126-1 and 126-2 are configured, therefore, to force optical mode216 substantially completely into coupling waveguide 112 by the timelight signal 118 reaches transition region 104.

In similar fashion, in the depicted example, w4 and w5 of passivewaveguide 114 are selected such that optical mode 216 is substantiallyconfined within its ridge and planar regions within transition region104. In some embodiments, however, passive waveguide 114 is configuredsuch that the optical energy of optical mode 216 extends across acontinuous region that occupies at least portions of both passivewaveguide 114 and silicon device layer 206. In other words, each ofpassive waveguide 114 and silicon device layer 206 partially supportsoptical mode 216.

FIG. 2C depicts a schematic drawing of a sectional view of transitionregion 104 as taken through line c-c shown in FIG. 1.

Coupler 128-2 is a section of output-coupling region 106 configured forefficiently optically coupling light signal 118 from passive waveguide114 into silicon waveguide 116. Coupler 128-2 includes taper 126-3 and asegment of silicon waveguide 116, where taper 126-3 is a segment ofpassive waveguide 114 that is configured to facilitate the transfer oflight signal 118 into silicon waveguide 116 as a single-mode signal.

FIG. 2D depicts a schematic drawing of a sectional view of coupler 128-2as taken through line d-d shown in FIG. 1. Note that the planar portionof silicon waveguide 116 is not shown in FIG. 1.

Taper 126-3 has length L3, over which the width of passive waveguide 114changes from w4 to extinction. As a result, taper 126-3 is configured toforce optical mode 216 completely into silicon waveguide 116. In thedepicted example, L3 is equal to 200 microns; however, it is typicallywithin the range of approximately 50 microns to approximately 1000microns. It should be noted that the value of L3 is a matter of designchoice and, therefore, it can have any suitable value. As discussedabove and with respect to taper 126-1, in some embodiments, taper 126-3does not taper to extinction but, rather, to a non-zero width (e.g., onemicron or less) that is sufficiently narrow to force the optical energyof light signal 118 from passive waveguide 114 into silicon waveguide116. Furthermore, in some embodiments, silicon waveguide 116 includes ataper that facilitates transfer of optical energy between the siliconand passive waveguides. In some embodiments, both passive waveguide 114and silicon waveguide 116 include a taper.

FIG. 2E depicts a schematic drawing of a sectional view of siliconwaveguide 116 as taken through line e-e shown in FIG. 1. It should benoted that, in the depicted example, the regions between siliconwaveguide 116 and taper 126-3 are air-cladding regions; however, theseregions can be filled with any material suitable to function as claddingmaterial for the silicon waveguide, such as silicon dioxide, siliconnitride, polymer, and the like.

It is another aspect of the present invention that OA device 110 can beconfigured to support an optical mode that is discontinuous such that,at some points within active region 102, it includes separateoptical-mode portions that propagate together but are distributed amongactive-material stack 210 and coupling waveguide 114 and, in someembodiments, silicon device layer 206. In other words, each ofactive-material stack 210 and coupling waveguide 114 and silicon devicelayer 206 partially supports optical mode 216 by supporting a differentone of its optical-mode portions.

FIG. 5A depicts a schematic drawing of a sectional view of analternative composite waveguide in accordance with the presentdisclosure. The sectional view depicted in FIG. 5A is taken through aregion analogous to that intersected by line a-a shown in FIG. 1.Composite waveguide 500 includes substrate 108, coupling waveguide 502,and active-material stack 504. For clarity, electrical contacts are notshown in FIG. 5A.

In composite waveguide 500, coupling waveguide 502 and active-materialstack 504 are configured such that each includes at least one sub-layerthat is configured to force optical energy of optical mode 216 into adifferent sub-layer of that element. As a result, optical mode 216 issplit into two discontinuous optical-mode portions—optical-mode portions216A and 216B. In other words, each of coupling waveguide 502 andactive-material stack 504 partially supports optical mode 216 bysupporting a different one of optical-mode portions 216A and 216B.

Coupling waveguide 502 is analogous to coupling waveguide 112; however,coupling waveguide 502 has width w8 and includes sub-layers 502A, 502B,and 502C, where each of sub-layers 502A and 502C has a refractive indexthat is lower than that of sub-layer 502B. As a result, sub-layers 502Aand 502C function as lower and upper cladding layers, respectively, thatsubstantially confine the bulk of the optical energy of optical-modeportion 216A to sub-layer 502B.

Active-material stack 504 is analogous to active-material stack 210;however, active-material stack 504 has width w7 and includes sub-layers506A and 506B, each of which has a refractive index that is higher thanthat of gain layer 212. As a result, sub-layers 506A and 506B functionas upper and lower cladding layers, respectively, that substantiallyconfine the bulk of the optical energy of optical-mode portion 216B tothe portion of active-material stack 504 that resides between them.

The widths of optical-mode portions 216A and 216B and the spacingbetween them are based upon widths w7 and w8 and the sub-layerconfigurations of coupling waveguide 502 and active-material stack 504.

In some embodiments, coupling waveguide 502 and active-material stack504 are configured such that optical mode 216 includes a thirdoptical-mode portion that is located in silicon device layer 206.

FIG. 5B depicts a schematic drawing of a sectional view of anotheralternative composite waveguide in accordance with the presentdisclosure. The sectional view depicted in FIG. 5B is taken through aregion analogous to that intersected by line a-a shown in FIG. 1.Composite waveguide 508 is analogous to composite waveguide 500;however, in composite waveguide 508, coupling waveguide 502 andactive-material stack 504 are configured to split the optical energy ofoptical mode 216 into three discontinuous optical-modeportions—optical-mode portions 216C, 216D, and 216E.

Coupling waveguide 510 is analogous to coupling waveguide 502; however,coupling waveguide 510 has width w10 and includes sub-layers 502D, 502E,and 502F, where each of sub-layers 502D and 502F has a refractive indexthat is lower than that of sub-layer 502E. As a result, sub-layers 502Dand 502F function as lower and upper cladding layers, respectively, thatsubstantially confine the bulk of the optical energy of optical-modeportion 216D to sub-layer 502E.

Active-material stack 512 is analogous to active-material stack 504;however, active-material stack 512 has width w9 and includes sub-layers514A and 514B, each of which has a refractive index that is lower thanthat of gain layer 212. As a result, sub-layers 514A and 514B functionas upper and lower cladding layers, respectively, that substantiallyconfine the bulk of the optical energy of optical-mode portion 216C tothe portion of active-material stack 512 that resides between them.

In the depicted example, coupling waveguide 510 and active-materialstack 512 are further configured to give rise to additional optical-modeportion 216E, which is discontinuous with optical-mode portions 216C and216D and substantially confined to silicon device layer 206.

As a result, each of coupling waveguide 510, active-material stack 512,and silicon device layer 206 partially supports optical mode 216 bysupporting a different one of its optical-mode portions. The widths ofoptical-mode portions 216C, 216D, and 216E, as well as the spacingbetween them, are based upon widths w9 and w10 and the sub-layerconfigurations of coupling waveguide 510 and active-material stack 512.

In some embodiments, coupling layer 208 is configured such that opticalmode 216 is split into discontinuous optical-mode portions in transitionregion 104, with one of the optical-mode portions being located inpassive waveguide 112 and another optical-mode portion is located insilicon device layer 206.

FIG. 6 depicts a schematic drawing of a sectional view of an alternativetransition region in accordance with the present disclosure. Thesectional view depicted in FIG. 6 is taken through a region analogous tothat intersected by line c-c shown in FIG. 1. Transition region 600includes substrate 108, passive waveguide 602, and cladding layer 604.Transition region 600 is analogous to transition region 104 describedabove; however, in transition region 600, optical mode 216 includesdiscontinuous optical-mode portions 216E and 216F, which reside inpassive waveguide 602 and silicon device layer 206, respectively.

Passive waveguide 602 is analogous to passive waveguide 114; however,passive waveguide 602 includes sub-layer 606, which has a refractiveindex that is higher than the remainder of the passive waveguide.

Cladding 604 is a thin layer of material suitable for substantiallyblocking the passage of optical energy of light signal 118 betweenpassive waveguide 114 and silicon device layer 206. In the depictedexample, cladding 604 is a layer of silicon dioxide having a thicknessof approximately 100 nm; however, it will be clear to one skilled in theart, after reading this Specification, how to specify, make, and usealternative embodiments wherein cladding 604 comprises a differentmaterial and/or has a different thickness.

As noted above, passive waveguide 602, cladding 604 and sub-layer 606are configured such that they collectively support optical mode 216,which includes discontinuous optical-mode portions 216F and 216G, whichare located in passive waveguide 114 and silicon device layer 206,respectively (i.e., passive waveguide 114 partially supports opticalmode 216 by supporting optical-mode portion 216F and silicon devicelayer 206 partially supports optical mode 216 by supporting optical-modeportion 216G). The shapes, vertical positions, and separation betweenoptical-mode portions 216F and 216G are based on the values of w11 thatlocation.

FIG. 7 depicts a schematic drawing of a sectional view of an alternativeembodiment of a coupler for optically coupling a passive waveguide and asilicon waveguide in accordance with the present disclosure. Thesectional view depicted in FIG. 7 is taken through a region analogous tothat intersected by line d-d shown in FIG. 1. Coupler 700 includessubstrate 108, silicon waveguide 116, cladding 702, and taper 704.

Cladding 702 is analogous to cladding 604.

Taper 704 is analogous to taper 126-3; however, taper 704 has widthw12(x) along length L3 and includes sub-layer 706, which has arefractive index that is higher than the remainder of the taper. As aresult, sub-layer 706 serves to confine optical energy of light signal118 to the region of taper 704 located between cladding 702 andsub-layer 706.

By virtue of cladding 702 and sub-layer 706, optical mode 216 includesdiscontinuous optical-mode portions 216H and 216J, which are located inand supported by silicon waveguide 116 and taper 704, respectively. As aresult, each of silicon waveguide 116 and taper 704 partially supportsoptical mode 216. The shapes, vertical positions, and separation betweenoptical-mode portions 216H and 216J at any location along length L3 arebased on the values of w12(x) and w6 at that location and theconfiguration of taper 704 and sub-layer 706.

In some embodiments, light signal 118 is optically coupled betweenpassive waveguide 114 and silicon waveguide 116 via a turning reflectorand vertical grating coupler.

FIG. 8 depicts a schematic drawing of a cross-sectional view of yetanother alternative coupler in accordance with the present disclosure.Coupler 800 includes reflector 802 and vertical grating coupler 804.

Reflector 802 is an angled facet formed in passive waveguide 114.Reflector 802 is configured to receive light signal 118 propagatingalong longitudinal axis A1 of the passive waveguide and redirect italong a direction that is substantially normal to axis A1. In someembodiments, reflector 802 includes one or more surface layers (e.g.,metals, dielectrics, etc.) for improving its reflectivity for lightsignal 118.

Grating 804 is a vertical grating coupler formed in silicon waveguide116 and configured to receive light signal 118 from reflector 802 andredirect it along longitudinal axis A2 of the silicon waveguide.

As will be apparent to one skilled in the art, a vertical gratingcoupler typically has a range of angles at which light signal 118 can bereceived and successfully coupled into a waveguide. As a result,reflector 802 can be configured to redirect the light signal along anyangle within the acceptance range of grating 804.

In some embodiments, grating 804 is formed in passive waveguide 114 andreflector 802 is formed in silicon waveguide 116.

In some embodiments, silicon waveguide includes an output portcomprising a reflector configured to launch light signal 118 out of theplane of the silicon waveguide as a free-space signal.

FIGS. 9A and 9B depict schematic drawings of cross-sectional views ofalternative output ports in accordance with the present disclosure. Eachof ports 900 and 902 include reflector 904, which is formed in siliconwaveguide 116.

Reflector 904 is analogous to reflector 802 described above; however,reflector 904 is formed in silicon waveguide 116 to redirect lightsignal 118 as indicated.

Port 900 includes reflector 904 and facet 906. Facet 906 is formed insilicon waveguide 116 such that light signal 118 is launched into freespace and received by reflector 904. Reflector 904 is configured toredirect free-space light signal 118 away from handle substrate 202.

Port 902 includes reflector 904 is configured to launch light signal 118as a free-space signal directed toward substrate 908.

Substrate 908 is analogous to substrate 108 described above; however, itis preferable that substrate 908 comprise a material that issubstantially transparent and non-absorptive for the wavelengths oflight signal 118 so that the light signal can pass completely throughthe substrate with little or no attenuation.

While it is preferable in most applications to optically couple lightsignal 118 between OA device 110 and silicon waveguide 116, in someembodiments, the light signal is provided to, or received from, anexternal device or system without being coupled into the siliconwaveguide.

FIG. 10 depicts a schematic drawing of a cross-sectional view of analternative system in accordance with the present disclosure. System1000 is analogous to system 100; however, in system 1000, light signal118 is launched into free space directly from passive waveguide 114.System 1000 is disposed on substrate 1002 and includes facet 1004 andbulk reflector 1006.

Substrate 1002 is a bulk silicon substrate suitable for use in planarprocessing.

Facet 1004 is an end facet formed in passive waveguide 114 inconventional fashion (e.g., by etching, dicing, partial dicing, etc.).At facet 1004, light signal 118 exits the passive waveguide asfree-space signal 1008.

Reflector 1006 is analogous to reflector 804; however, reflector 1006 isa bulk reflector mounted on substrate 1002 such that it receivesfree-space signal 1008 from passive waveguide 114. In some embodiments,reflector 1006 is formed in a region of coupling layer 208 outside ofthe area of passive waveguide 114.

In some embodiments, it is preferable to precisely locate a bulk opticalelement to receive free-space signal 1008 directly from passivewaveguide 114.

FIG. 11 depicts a schematic drawing of a cross-sectional view of anotheralternative system in accordance with the present disclosure. System1100 is analogous to system 1000; however, system 1100 includes bulkoptical element 1102 and alignment feature 1104. Element 1102 isprecisely located on substrate 1002 by alignment feature 1104 such thatit receives light signal 118 from passive waveguide 114.

In the depicted example, element 1102 is an optical fiber; however,element 1102 can include a wide range of device and systems withoutdeparting from the scope of the present disclosure. Devices and systemssuitable for use in element 1102 include, without limitation, opticalfibers, PICs, photodetectors, light sources (edge-emitting lasers,vertical-cavity surface-emitting lasers (VCSELs), light-emitting diodes,integrated-optics systems, planar-lightwave circuits (PLCs), and thelike.

Alignment feature 1104 is a channel etched in substrate 1002 viaconventional methods (e.g., reactive-ion etching,crystallographic-dependent etching, ion milling, laser-assisted etching,etc.) such that its depth aligns the core of element 1102 with passivewaveguide 114. In some embodiments, alignment feature 1104 includes atleast one projection disposed on the top surface of substrate 1002 (orsilicon device layer 206 in embodiments where system 1100 is disposed onan SOL such as substrate 108), where the projection is configured toconstrain element 1102 in at least one dimension.

It is to be understood that the disclosure teaches only examples ofembodiment in accordance with the present disclosures and that manyvariations of these embodiments can easily be devised by those skilledin the art after reading this disclosure and that the scope of thepresent invention is to be determined by the following claims.

What is claimed is:
 1. An integrated-optics system disposed on asubstrate, the system comprising an optically active device that isoptically coupled with a silicon waveguide, the system including: afirst region that selectively includes the optically active devicecomprising an active-material stack the active-material stack beingdisposed on a coupling waveguide that includes a first segment of afirst layer that comprises a compound semiconductor, wherein theoptically active device and the coupling waveguide collectively define acomposite waveguide that at least partially supports an optical mode ofa light signal; a second region that includes a first waveguide that isa passive waveguide configured to at least partially support the opticalmode, the first waveguide having a first core that comprises a secondsegment of the first layer, the first waveguide being optically coupledwith the composite waveguide; and a third region that selectivelyincludes a second waveguide having a second core comprisingsingle-crystal silicon, wherein the second waveguide is configured to atleast partially support the optical mode and is optically coupled withthe first waveguide; wherein the first region, second region, and thirdregion are contiguous.
 2. The system of claim 1 wherein, in the firstregion, the coupling waveguide is disposed on a second layer comprisingsingle-crystal silicon, and wherein the optical mode extends over acontinuous region that includes at least a portion of each of theactive-material stack, the coupling waveguide, and the second layer. 3.The system of claim 1 wherein, in the second region, the passivewaveguide is disposed on a second layer comprising single-crystalsilicon, and wherein the optical mode extends over a continuous regionthat includes at least a portion of each of the passive waveguide andthe second layer.
 4. The system of claim 1 wherein, in the secondregion, the passive waveguide is disposed on a second layer comprisingsingle-crystal silicon, and wherein the optical mode includes a firstoptical-mode portion and a second optical-mode portion, the first andsecond optical-mode portions being discontinuous, and wherein the firstoptical-mode portion is located in the passive waveguide and the secondoptical-mode portion is located in the second layer.
 5. The system ofclaim 1 wherein, in the first region, the optical mode includes a firstoptical-mode portion and a second optical-mode portion, the first andsecond optical-mode portions being discontinuous, and wherein the firstoptical-mode portion is located in the optically active device and thesecond optical-mode portion is located in the coupling waveguide.
 6. Thesystem of claim 5 wherein, in the first region, the coupling waveguideis disposed on a second layer that comprises single-crystal silicon andthe optical mode further includes a third optical-mode portion that islocated in the second layer, and wherein the first, second, and thirdoptical-mode portions are discontinuous.
 7. The system of claim 6wherein the substrate is a silicon-on-insulator substrate that includesa handle substrate, a buried-oxide layer, and a device layer, andwherein the second layer is the device layer, and further wherein thesecond core comprises at least a portion of the second layer.
 8. Thesystem of claim 1 wherein the optically active device includes at leastone quantum element, and wherein the at least one quantum element isselected from the group consisting of a quantum dot, a quantum well, aquantum-well layer, a quantum dash, and a quantum wire.
 9. The system ofclaim 1 wherein the optically active device includes a first taperhaving a first length and a lateral dimension that changes from a firstwidth to a second width along the first length, and wherein at least oneof the size and the vertical position of the optical mode at eachlocation along the first length is based on the lateral dimension atthat location.
 10. The method of claim 9 wherein the second width isequal to zero.
 11. The system of claim 1 further comprising a firstreflector that is configured to receive the light signal along a firstdirection and redirect the light signal along a second direction that isunaligned with the first direction.
 12. The system of claim 11 furthercomprising a vertical-grating coupler configured to receive the lightsignal from the first reflector, the first reflector being located inthe first waveguide and the vertical grating coupler being located inthe second waveguide.
 13. The system of claim 11 wherein the firstreflector is located in one of the first waveguide and the secondwaveguide.
 14. The system of claim 11 wherein the first reflector isconfigured to receive the light signal from one of the first waveguideand second waveguide as a free-space light signal.
 15. The system ofclaim 1 further comprising a first element and a first alignment featurefor positioning the first element relative to one of the first waveguideand the second waveguide such that the first element and the OA deviceare optically coupled, wherein the first element is selected from thegroup consisting of an optical fiber, a surface waveguide, an opticaldetector, a laser, an optical amplifier, and a light-emitting diode. 16.The system of claim 1 wherein the optically active device is selectedfrom the group consisting of a laser, an optical modulator, an opticalamplifier, a variable optical attenuator, and a photodetector.
 17. Thesystem of claim 1 wherein the passive waveguide includes a first taperhaving a first length and a lateral dimension that changes from a firstwidth to a second width along the first length, and wherein at least oneof the size and the vertical position of the optical mode at eachlocation along the first length is based on the lateral dimension atthat location.
 18. The method of claim 17 wherein the second width isequal to zero.
 19. The system of claim 1 wherein the first layer has afirst sub-layer that has a first refractive index and a second sub-layerthat has a second refractive index that is higher than the firstrefractive index, the second sub-layer being between the first sub-layerand the optically active device, and wherein the optically active deviceincludes a third sub-layer that has a third refractive index that islower than the second refractive index.
 20. A method for forming anintegrated-optics system disposed on a first substrate, the systemhaving a first region, a second region, and a third region andcomprising an optically active device that is optically coupled with asilicon waveguide, the method including: forming a coupling waveguide inthe first region, the coupling waveguide including a first segment of afirst layer comprising a compound semiconductor; forming an opticallyactive device that includes an active-material stack, theactive-material stack being disposed on the coupling waveguide, whereinthe optically active device and the coupling waveguide collectivelydefine a first composite waveguide that at least partially supports anoptical mode of a light signal, and wherein the optically active deviceis located only in the first region; forming a first waveguide in asecond region, the first waveguide being a passive waveguide configuredto at least partially support the optical mode, wherein the firstwaveguide has a first core that comprises a second segment of the firstlayer, and wherein the first waveguide is optically coupled with thefirst composite waveguide; and forming a second waveguide in a thirdregion, the second waveguide having a second core comprisingsingle-crystal silicon and being configured to at least partiallysupport the optical mode, wherein the second waveguide is opticallycoupled with the first waveguide.
 21. The method of claim 20 wherein, inthe first region, the optical mode includes a first optical-mode portionand a second optical-mode portion, the first and second optical-modeportions being discontinuous, and wherein the optically active deviceand the coupling waveguide are formed such that the first optical-modeportion is located in the optically active device and the secondoptical-mode portion is located in the coupling waveguide.
 22. Themethod of claim 20 wherein, in the first region, the coupling waveguideis disposed on a second layer that comprises single-crystal silicon andthe optical mode further includes a third optical-mode portion that islocated in the second layer, and wherein the first, second, and thirdoptical-mode portions are discontinuous.
 23. The method of claim 20wherein the first substrate is provided as a silicon-on-insulatorsubstrate that includes a handle substrate, a buried-oxide layer, and adevice layer, and wherein the second layer is the device layer, andfurther wherein the second waveguide is formed such that the second corecomprises at least a segment of the second layer.
 24. The method ofclaim 20 wherein, in the first region, the coupling waveguide isdisposed on a second layer comprising single-crystal silicon, andwherein the optical mode extends over a continuous region that includesat least a portion of each of the optically active device, the couplingwaveguide, and the second layer.
 25. The method of claim 20 wherein, inthe second region, the passive waveguide is disposed on a second layercomprising single-crystal silicon, and wherein the optical mode extendsover a continuous region that includes at least a portion of each of thepassive waveguide and the second layer.
 26. The method of claim 20wherein, in the second region, the passive waveguide is disposed on asecond layer comprising single-crystal silicon, and wherein the opticalmode includes a first optical-mode portion and a second optical-modeportion, the first and second optical-mode portions being discontinuous,and wherein the first optical-mode portion is located in the passivewaveguide and the second optical-mode portion is located in the secondlayer.
 27. The method of claim 20 further comprising: epitaxiallygrowing the first layer on a second layer that comprises single-crystalsilicon, the first substrate including the second layer; epitaxiallygrowing the active-material stack on the first layer; patterning thefirst layer and the active-material stack in the first region to definethe first composite waveguide; and forming a plurality of electricalcontacts that are operatively coupled with the active-material stack.28. The method of claim 20 wherein the optically active device is formedsuch that it includes at least one quantum element that is selected fromthe group consisting of a quantum dot, a quantum well, a quantum-welllayer, a quantum dash, and a quantum wire.
 29. The method of claim 20wherein the optically active device is selected from the groupconsisting of a laser, an optical modulator, an optical amplifier, avariable optical attenuator, and a photodetector.
 30. The method ofclaim 20 further comprising: epitaxially growing the first layer and theactive-material stack on a second substrate; joining the first layer anda second layer, the first substrate comprising the second layer, whereinthe second layer comprises a first material selected from the groupconsisting of single-crystal silicon, silicon oxides, silicon nitrides,polymers, and metals; patterning the first layer and the active-materialstack in the first region to define the first composite waveguide; andforming a plurality of electrical contacts that are operatively coupledwith the active-material stack.
 31. The method of claim 20 furthercomprising patterning the active-material stack in the first region suchthat the optically active device has a first taper having a first lengthand a first lateral dimension that changes from a first width to asecond width along the first length, wherein at least one of the sizeand the vertical position of the optical mode in the first compositewaveguide at each location along the first length is based on the firstlateral dimension at that location.
 32. The method of claim 31 whereinthe second width is equal to zero.
 33. The method of claim 20 furthercomprising forming a first reflector that is configured to receive thelight signal along a first direction and redirect the light signal alonga second direction that is unaligned with the first direction.
 34. Themethod of claim 33 further comprising forming a vertical grating couplerconfigured to receive the light signal from the first reflector, thefirst reflector being located in the first waveguide and the verticalgrating coupler being located in the second waveguide.
 35. The method ofclaim 33 further comprising forming the first reflector such that it islocated in one of the first waveguide and the second waveguide.
 36. Themethod of claim 33 further comprising forming the first reflector suchthat it is configured to receive the light signal from one of the firstwaveguide and second waveguide as a free-space light signal.
 37. Themethod of claim 20 further comprising forming a first alignment featurethat is configured to vertically position a first element relative toone of the first waveguide and the second waveguide to enable the lightsignal to optically couple into the first element, wherein the firstelement is selected from the group consisting of an optical fiber, asurface waveguide, an optical detector, a laser, an optical amplifier,and a light-emitting diode.
 38. The method of claim 20 wherein the firstregion is formed such that the first layer has a first sub-layer thathas a first refractive index and a second sub-layer that has a secondrefractive index that is higher than the first refractive index, thesecond sub-layer being between the first sub-layer and the opticallyactive device, and wherein the optically active device includes a thirdsub-layer that has a third refractive index that is lower than thesecond refractive index.
 39. The method of claim 20 wherein forming thesecond region includes patterning the first layer to define the firstcore.
 40. The method of claim 20 wherein forming the third regionincludes operations comprising: patterning a second layer to define thesecond core, wherein the second layer comprises single-crystal silicon;and patterning the first layer to define a taper having a first lengthand a lateral dimension that changes from a first width to a secondwidth along the first length, wherein the first layer and second layerare in contact along at least a portion of the first length; wherein atleast one of the size and vertical position of the optical mode at eachlocation along the first length is based on the lateral dimension atthat location.
 41. The method of claim 40 wherein the second width isequal to zero.